1. Field of the Invention
The present invention relates to a process for treating lignocellulose pulp using a preparation containing at least one enzyme that produces a solubilizing action and, potentially, a hemicellulolytic action on lignin. The enzymes are derived from the bacterium Streptomyces viridosporus.
This enzymatic treatment may be used for the separation of lignin from lignocellulose pulp in a procedure, for example, that complements a bleaching treatment.
2. Background Information
The present invention relates to the enzymatic treatment of chemical lignocellulose pulps used in the paper industry.
These pulps are obtained from wood, which is composed essentially of a fibrous matrix of cellulose and hemicellulose, as well as lignin, whose complex macromolecular structure is similar to the fibrous matrix.
The presence of lignin causes certain difficulties. In particular, lignin is sensitive to ultraviolet light, causing formation of brownish products which are not highly valued and which impair the whiteness normally sought in white paper products.
For this reason, the paper industry attempts to eliminate all or a portion of the lignin. This elimination process utilizes chemical treatments; thus, the pulp obtained is a chemical pulp.
When white paper is produced, supplementary treatments, called bleaching treatments, prove necessary. These processes generally encompass several steps, including, in particular, delignification under oxygen (step O); chlorination procedures using chlorine Cl.sub.2 (step C), chlorine dioxide (step D), or a mixture of the two (steps C/D), where the percentage of substitution of ClO.sub.2 for Cl.sub.2 may reach 50 to 100%; and hypochlorite-reaction steps (steps H).
In addition, alkaline-extraction procedures (steps E), potentially enhanced under oxygen (steps (E/O), may also be implemented.
Thus, currently-used bleaching procedures involve the following steps in succession using the abbreviations designated above as:
C-E-H-D or C/D-E-D-E-D or O-C/D-E/O-D-E-D.
However, this chlorine reaction procedure produces by-products such as chlorolignins which biodegrade only slightly and may be toxic, and which at certain concentrations may pollute the environment when drained away as effluents.
Alternative procedures are now available which use, for example, ozone and/or hydrogen peroxide; however, the whiteness of the resulting paper needs further improvement.
The problems addressed by Applicants and others in the field were to obtain a pulp having at least the equivalent degree of whiteness using fewer chlorinating agents when bleaching procedures containing chlorinating agents were used, and in cases of bleaching procedures where chlorinating agents were not utilized, to improve the final whiteness in the product.
Others in the field have examined the use of enzymatic preparations derived from microorganisms in treatments that separate lignin from the lignocellulose pulp. The principal part of these protocals focusing on the fungal enzymes are described below.
Patents U.S. Pat. No. 4,690,895 and EP-A-345 715 both describe delignification of kraft pulps using an enzymatic preparation derived from the fungus Phanerochaete chrysosporium.
U.S. Pat. No. 4,690,895 describes a mutant of this fungus which produces lignases, specifically, peroxidases which makes it possible to decompose the aromatic compounds in the lignin.
The treatment requires pulp to be in a solution having an acidic pH of between 2 and 7, and preferably 4.5, enriched with oxygen and fed with hydrogen peroxide in the presence of manganese sulfate. Incubation lasts for 12 h at 39.degree. C.
A second incubation using sodium carbonate must then be carried out before decanting the pulp and washing it in water.
In Patent Application EP-A-345 715, the enzymes described are also lignins-peroxidases and/or lignins-peroxidases combined with manganese which has a degree of oxidation of 2 (Mn II). The enzymatic conditions in this treatment are similar to U.S. Pat. No. 4,690,895, the difference being the additional presence of a detergent and another compound such as an .alpha.-hydroxylated acid. The preferred manner of implementing the procedure consists in an alkaline extraction, washing in water, followed by an acid extraction.
Patent Application EP-A-371 712 studies the gene responsible for the formation of the lignin-peroxidase enzyme in the DNA structure of the bacterium Streptomyces viridosporus. According to this application, the enzyme makes it possible to oxidize the lignin in the presence of hydrogen peroxide having an acidic pH.
In accordance with a slightly different procedure, Patent Application EP-A-373 107 proposes a treatment using hemicellulases which are enzymes derived form the fungus Aureobasidium pullulans. Hemicellulase displaces the lignin which is covalently bonded by decomposing the hemicellulase and the lignin is separated by alkaline extraction. The treatment may also be carried out at an acidic pH, i.e., of approximately 5.
In a similar manner, Patent Applications EP-A-383 999 and EP-A-395 792 propose the use of enzymes in general to treat the pulps, and in particular, hemicellulases derived from bacteria belonging to the Streptomyces family are disclosed.
However, these methods proposing treatment with hemicellulase may cause problems. Hemicellulase has no specific action on lignin and tends to decompose not only the hemicellulose bonded covalently to the lignin, but also acts on hemicelluloses not bonded to the lignin, which is not the objective sought. The portion of the hemicellulose not bonded to the lignin but yet solubilized by these enzymes may cause a certain reduction in yield. Furthermore, it may increase the biochemical oxygen demand (BOD) of the reactive medium, a portion of which becomes effluents in the environment.
Thus, it appears that the only fungal or bacterial enzymes known for their specific action on lignin are peroxidases. The treatment of a lignocellulose pulp using preparations based on such enzymes implies the obligatory presence of a co-substrate such as hydrogen peroxide, and even the presence of additional agents.
The only alternative is to use hemicellulases, whose disadvantages have already been mentioned.
Furthermore, it has been found that in most of the types of enzymatic preparations mentioned above, a substantial acidic pH is recommended. But, after undergoing alkaline chemical treatment during baking and/or bleaching, the pulps have a basic pH greater than 8 in aqueous suspension. This implies that if one desires to treat the pulp after an alkaline treatment of this kind, a preliminary acidification step must be carried out.
The object of the present invention is to provide an enzymatic method for treating chemical lignocellulose pulps in order to separate out the lignin. The present invention makes it possible to eliminate the disadvantages of the aforementioned enzymatic treatment through a simplified implementation procedure which requires, in particular, neither preliminary acidification nor co-substrate.